1. Field of the Invention
The present invention relates to a brushless motor and an electric power steering system which includes the brushless motor to assist the driver in steering a vehicle in such a way as to control the driving of the brushless motor according to a steering torque detected by a torque sensor.
2. Related Art
An electric power steering system for reducing load borne by the driver in steering a vehicle is such as to include a steering member (a steering wheel, steering handle) which is connected to an input shaft, an output shaft which is connected to control wheels via a pinion, a rack and the like and a connecting shaft which connects the input shaft with the output shaft, whereby a steering torque applied to the input shaft is detected by a torque sensor from a twist angle generated in the connecting shaft, so as to control the driving of a steering assist motor which is linked with the output shaft based on the torque value so detected.
In recent years, brushless motors have come to be used as steering assist motors in such electric power steering systems in order to cope with the tendency of high output. A brushless motor is a motor which includes permanent magnets on a rotor and controls a waveform forming circuit for generating a rotating magnetic field in a stator based on the (rotational) position of the rotor so as to cause the rotor to rotate, and since no brush is used therein, a loss by brushes, mechanical noise and electric noise by the brushes are not produced.
JP-A-2003-250254 discloses a permanent magnet type brushless motor for an electric power steering system which has ten permanent magnets on a rotor and twelve slots between teeth on a stator.
In a steering assist brushless motor for an electric steering system, for example, when teeth on a stator core are made thin, saturation in magnetic flux density is generated, and the linearity of a current-torque function is collapsed largely by the effect of the saturation in magnetic flux density, leading to a problem that the steering assist control becomes difficult.